The subject says it all. Complete a quick quiz, they send you a free sample of the wash-my-jacket-in-a-hostel-sink size. Great for traveling just in case you do a half gainer into a late season mud patch.
They even give you the answers to the quiz click 'check my answers'
Nikwax Web Quiz - Win Nikwax Product(s)
I've used it. The stuff works.
Obviously, if you want the sample to be sent, you've gotta give 'em your addy.